This patent relates to the formation of diaphragm based microstructures used primarily for sensors at this time, in which the diaphragm may be used by sensing a change in deflection, resonance, or curvature as a result of environmental influences which are allowed to reach the sensor diaphragm. Particular sensors are described which may be constructed from the method of manufacture. The method described and structures shown are primarily silicon based, either single crystal form or polycrystalline in form, but other materials could also be used. Recently there has been a substantial interest in microstructures. A survey of structures and methods was published in Scientific American in April 1983, Angel et al, "Silicon Micromechanical Devices". The article describes a sensor similar to the pressure sensor of FIG. 1 of this patent, constructed through micromachining or chemical etching including "pits, holes, pyramids, trenches, hemispheres, cantilevers, diaphragms, needles and walls".
However, it was found that there are process reliability problems, particularly with producing diaphragm structures. Also, there is now a desire to produce more complex diaphragm structures. For example, where a wafer having a diaphragm on one side is placed over another wafer with a corresponding cavity below the diaphragm, bonding the two wafers at high temperatures may cause the diaphragm to permanently deform due to expansion of gases trapped on one side but not on the other. Attempting to provide a bond by merely contacting the two wafers having clean surfaces, is a well known method, and it is also described in U.S. Pat. No. 4,638,552 but it does not produce a satisfactory strong bond for the purpose of producing reliable sensors. In contrast, it has long been known that high temperature bonding of silicon can produce a strong bond.
Diaphragm-based sensors usually have within them electrical sensor elements, such as piezoresistors or capacitor elements, and these elements, once fabricated, can be damaged by subsequent high temperature processing. An additional problem thus arises in the fabrication of multiple wafer diaphragm structures, where the bonding of some wafers must occur after the formation of such electrical sensor elements.
One approach to manufacturing steps for diaphragm microstructures are taught in European Patent Application 89304173.1, entitled "Laminated Semiconductor Sensor With Overpressure Protection", filed Apr. 26, 1989. The process described in it, however, not only does not protect the diaphragm during processing, but adds the piezoresistive sensors before the high temperature fusing of the wafers and therefore creates a risk that these resistors or other sensitive electrical components will also be damaged by subsequent processing. No process previously known teaches how to adequately protect the sensor through the bonding process.
Also, where it is useful or desired to have a very flat or polished surface in which one surface of the diaphragm forms a contiguous part thereof, there has heretofore not been known a satisfactory method for polishing, without causing damage to the diaphragm.
Other known methods for building diaphragm based microsensors are either more difficult manufacturing processes than fusion bonding of wafers as shown in the European Application No. 89304173.1 (above-mentioned), or do not yield the same result sought here. An example of a "depositing" method can be found in U.S. Pat. No. 4,744,863 issued to Guckel and Burns. Their work was used in U.S. Pat. No. 4,592,238 by Busta. The Guckel/Burns process, as related by Busta, employs "post" oxide (column 2, line 50 et seq) which is deposited onto the substrate layer and the diaphragm (polycrystalline silicon) is deposited onto this post oxide. The post oxide is later removed by leaving a diaphragm supported by deposited material only along its edges.
As an additional form of protection for sensitive electrical elements, this invention employs a low temperature electrostatic bonding process after the creation of such elements without using backfill in situations where the diaphragm itself is not likely to be too damaged.
Certain enhancements to the design and method of manufacture of these diaphragm sensors is also described herein. These features enhance the flexibility of uses to which the sensors can be put.